1. Field of the Invention
The present invention relates to an improved cryogenic distillation process for the production of oxygen and nitrogen and the integration thereof with a gas turbine.
2. Description of the Background
A conventional process for cryogenic separation of air components entails two distillation columns, one being a higher pressure column having the upper end in heat exchange relation with the lower end of a lower pressure column. Cold compressed air is separated into nitrogen-rich and oxygen-rich liquids in the higher pressure column. Thereafter, these liquids are fed to the lower pressure column to yield an oxygen product and a nitrogen product. This process is most efficient when the oxygen pressure is relatively low, on the order of about 1 to 2 bar.
U.S. Pat. No. 4,224,045 describes the use of air derived from a power turbine as air feed for an oxygen plant using a conventional two column process. Since the optimum air pressure of a power turbine is quite high, such as 10 to 18 bar absolute, the conventional/classical double column process must operate under elevated pressure in the high pressure column and low pressure column, thus, producing oxygen and nitrogen under pressures of 2 to 7 bar absolute. Relatively good overall efficiency is achieved when the nitrogen product as well as the oxygen product are recompressed to higher pressure for further integration with the associated process, such as coal gasification, and direct reduction for steel making or power generation, for example.
Unfortunately, this process has a major drawback in that the oxygen recovery is quite poor when the double column process is operated at elevated pressure. For example, an air pressure of 16 bar will yield an oxygen recovery of about 90% for 95% oxygen purity and about 5 bar absolute oxygen pressure. For 98% oxygen content, the recovery drops to about 80% for the same air and oxygen pressure. Conventional air separation plants operated at 6 bar absolute air pressure typically yield a recovery in excess of 99% of a purity of about 99.5%. The low recovery results in higher power consumption and larger equipment size.
U.S. Pat. No. 3,731,495 describes a two column apparatus and process for air separation with a nitrogen-quenched power turbine. In this process, air is separated by low temperature rectification using a 150-400 psia. column and a 45-140 psia. column with nitrogen-rich gas from the latter quenching hot combustion gas prior to work expansion of the resulting intermediate temperature gas mixture. However, as this process uses a double column, it suffers from the same disadvantages as that of U.S. Pat. No. 4,224,045.
U.S. Pat. No. 4,947,649 describes a single column process with a nitrogen recycle stream. In this process high pressure air is condensed at the bottom of the column and is fed directly to the column. The nitrogen product is compressed and a fraction is recycled back to the process to serve as additional reboil and reflux for the distillation column. The product recovery can be adjusted by varying the recycle flow rate. This process yields only a small improvement of about 1% in overall power consumption over the process disclosed in U.S. Pat. No. 4,224,045.
Thus, a need exists for a cryogenic distillation process and apparatus for the production and apparatus of oxygen and nitrogen at elevated air pressure. A need also exists for such a process and apparatus which can be used efficiently in combination with a gas turbine.